java - Using two LocalDateTime instances to calculate a duration -

i trying calculate duration between 2 instances of localdatetime. special thing here each instance of localdatetime anywhere in world:

localdatetime start nevada , localdatetime end tokyo. each "time" associated localdatetime is, enough, local location.

so if said...

localdatetime start = localdatetime.parse("2015-07-14t10:00:00"); , said start represented chicago, mean 10:00 in chicago time. say...

localdatetime end = localdatetime.parse("2015-07-14t03:00:00"); , end represents moscow, 3:00am in moscow time.

can create robust enough solution allow start , end represent any cities in world , still correctly calculate duration between two?

"localdatetime" not mean particular locality

i think misunderstand purpose of localdatetime. "local" means any locality, not specific locality. in "christmas starts @ midnight on december 25, 2015" mean any locality’s midnight. christmas starts in paris several hours earlier montréal, example.

if know date-time meant represent date-time in nevada, use joda-time datetime assigned proper time zone name of america/boise. in new java.time package (tutorial) built java 8 , later, use zoneddatetime object assigned time zone.

similarly, if know date-time local tokyo, not use localdatetime. use joda-time datetime assigned time zone of asia/tokyo.

elapsed

elapsed time between pair of localdatetime instances makes no sense. example, times may 14:00 , 18:00 on same date, not mean 4 hours difference. if meant 14:00 in paris , 18:00 in chicago, several hours difference, not two.

i not discussing calculating elapsed time has been handled many many times on stackoverflow. i'm trying clarify concepts here. can move on existing questions & answers calculating elapsed time.

databases store utc

generally in sql databases should using data type timestamp time zone (a.k.a. timestampz z "zulu"). misnomer means "with respect time zone". incoming data offset utc or other time zone information adjusted utc. data's offset or time zone not preserved.

the sql data type timestamp without time zone (a.k.a. timestamp) means same localdatetime in java: no time zone @ all. not tied timeline. offset or time zone information input data ignored, no adjustment made.

postgres doc may explain.

stick utc

when retrieving such value database, utc value may adjusted particular time zone admin tool (such pgadmin in postgres) or database driver or app.

in app best keep date-time values in utc as possible. of storage, business logic, , data exchange in utc. adjust particular time zone when expected user.

to convert localdatetime time zone

if have localdatetime object, , want assign time zone, here example code. adjust same moment seen in montréal , in utc. first example shown in joda-time, in java.time.

joda-time

example in joda-time 2.8.

localdatetime ldt = new localdatetime( "2015-07-14t10:00:00" );  // in particular. datetimezone zonechicago = datetimezone.forid( "america/chicago" ); datetime datetimechicago = ldt.todatetime( zonechicago ); datetime datetimemontreal = datetimechicago.withzone( datetimezone.forid( "america/montreal" ) ); datetime datetimeutc = datetimechicago.withzone( datetimezone.utc ); 

dump console.

system.out.println( "localdatetime (nowhere): " + ldt ); system.out.println( "chicago: " + datetimechicago ); system.out.println( "montréal: " + datetimemontreal ); system.out.println( "utc: " + datetimeutc); 

when run.

localdatetime (nowhere): 2015-07-14t10:00:00.000 chicago: 2015-07-14t10:00:00.000-05:00 montréal: 2015-07-14t11:00:00.000-04:00 utc: 2015-07-14t15:00:00.000z 

java.time

example in java.time of java 8 update 51.

localdatetime ldt = localdatetime.parse( "2015-07-14t10:00:00" );  // in particular. zoneid zonechicago = zoneid.of( "america/chicago" ); zoneddatetime zdtchicago = zoneddatetime.of( ldt, zonechicago ); zoneddatetime zdtmontreal = zdtchicago.withzonesameinstant( zoneid.of( "america/montreal" ) ); zoneddatetime zdtutc = zdtchicago.withzonesameinstant( zoneoffset.utc ); // zoneoffset subclass of zoneid. 

dump console.

system.out.println( "localdatetime (nowhere): " + ldt ); system.out.println( "chicago: " + zdtchicago ); system.out.println( "montréal: " + zdtmontreal ); system.out.println( "utc: " + zdtutc); 

when run.

localdatetime (nowhere): 2015-07-14t10:00 chicago: 2015-07-14t10:00-05:00[america/chicago] montréal: 2015-07-14t11:00-04:00[america/montreal] utc: 2015-07-14t15:00z